GB2325499A

GB2325499A - Friction clutch

Info

Publication number: GB2325499A
Application number: GB9815995A
Authority: GB
Inventors: Achim Link; Reinhold Weidinger
Original assignee: Mannesmann Sachs AG
Current assignee: ZF Friedrichshafen AG
Priority date: 1994-12-24
Filing date: 1995-12-22
Publication date: 1998-11-25
Anticipated expiration: 2015-12-22
Also published as: GB9815995D0; GB2325499B

Abstract

A friction clutch 1, with an automatic wear adjuster 12, has a rotatable clutch housing 3, a pressure plate 6 rotatable with the clutch housing 3 but axially moveable, a clutch plate 8 with, friction linings 8, a diaphragm spring 9 urging the pressure plate 6 to clamp the clutch plate 8 to a flywheel and a withdrawal system. A second spring 14 is arranged in the clutch to apply little or no force in a clutch release direction when the clutch is engaged and an increasing release force with increasing clutch withdrawal travel. The second spring 14 has inwardly extending spring tongues 21 through which it is connected to the diaphragm spring 9. Spring tongues 21 extend through gaps, corresponding at least to the thickness of the material of the second spring 14, in the diaphragm spring 9 and may be retained by hooks and a support ring (see fig 2). The clutch may be a push-to-release clutch or a pull-to-release clutch.

Description

FRICTION CLUTCH The invention relates to a friction clutch of the kind for use in a drive train of a motor vehicle and comprising a clutch housing secured to a flywheel of an internal combustion engine and rotatable with the flywheel about an axis, a pressure plate rotatable with the clutch housing and axially movable relative thereto, a clutch plate with friction linings arranged between the pressure plate and the flywheel, a diaphragm spring engaging the pressure plate and the clutch housing and urging the pressure plate towards the flywheel to generate an actuating force and a withdrawal element of a withdrawal system acting on the radially inner region of the diaphragm spring.

DE-C-39 91 022 shows a friction clutch of the kind set forth with an additional spring element acting with increasing wear of the friction linings to oppose the spring force of the diaphragm spring. This affects the increase in spring force, typical in a diaphragm spring in the wear region, so as to make substantially uniform the actuating force exerted by the diaphragm spring. This arrangement cannot reduce the actuating force; it simply ensures that the withdrawal forces exerted by the withdrawal system do not increase.

DE-C-944 050 shows a clutch actuating system in which an auxiliary spring is introduced as an over-centre spring through a number of intermediate levers and pivot points in such a way that a reduction in the actuating force takes place with increasing actuating travel of the clutch pedal. This construction is relatively expensive to manufacture and has a limited possibility of use.

It is an aim of the present invention to produce a reduction in the actuating force in a clutch of the kind set forth using the simplest possible means.

According to the present invention a friction clutch of the kind set forth includes an automatic adjuster for taking up the wear of the friction linings to maintain substantially constant an installed position of the diaphragm spring and the actuating force, and an annular spring member engaging an axially stationary component and another component of an actuating chain between and including the pressure plate and the withdrawal system, the annular spring member being so constructed and arranged to apply little or no force in a clutch release direction when the clutch is engaged, and an increasing release force with increasing clutch withdrawal travel.

The automatic adjuster maintains the installed position of the diaphragm spring and the actuating force while the annular spring member which, with the clutch engaged, applies little or no release force and with increasing withdrawal travel applies an increasing release force, allows an optimum association of the characteristic of the annular spring member with the characteristic of the diaphragm spring. This results in an effective reduction in the actuating force within the range of the withdrawal travel.

The annular spring member can be arranged at any point in the actuating chain. It may be a diaphragm spring or a plate spring.

Preferably the release force applied to the annular spring member during the withdrawal travel is not greater than the force applied to the withdrawal system by the diaphragm spring. This ensures that the actuating force of the diaphragm spring is maintained and the clutch pedal returns to its engaged position without external help, over the whole withdrawal travel, and thus over the whole actuating travel.

Advantageously the annular spring member engages against the clutch housing and directly against the diaphragm spring. This construction allows optimum matching or tuning during the manufacture and assembly of the clutch without having to take account of the influence of leverage ratios in the actuating system.

The annular spring member is preferably arranged substantially radially inwards of the radially inward engagement point of the diaphragm spring. In principle it is also possible to arrange the member radially outside the diaphragm spring but then it is necessary to match or fit the components, for example with respect to the clutch housing.

The annular spring member is preferably secured in the region of its two engagement points in the direction of the force exerted by it and in the opposite direction. This ensures that, even with the unavoidable spread of production tolerances that can lead to unfavourable relationships, the annular spring member is unable to snap over into the opposite force direction in the clutch engaged state. Accordingly on the next withdrawal sequence it is forcibly returned to its actuating position.

Where the clutch is a push-to-release clutch the diaphragm spring engages in the region of its outside diameter against the pressure plate through adjusting elements of the automatic wear adjuster and is pivotally mounted in an intermediate diameter region on the clutch housing through spacer pins, and the annular spring member has its outside diameter engaging in recesses in the spacer pins which have a gap of at least the thickness of the material of the annular spring member with respect to the inside wall of the clutch housing. The spacer pins, which are there in any case, therefore also perform the function of guiding the annular spring member in a direction opposite to the direction of the force which is normally applied.

Conveniently a radially inner region of the annular spring member is engaged by an engaging region of spring tongues of the diaphragm spring and the annular spring member has retaining elements engaging with the spring tongues to secure the spring member in the opposite direction. The annular spring member thus bears with its own engaging force against the inside of the clutch housing and against the spring tongues of the diaphragm spring and is secured against snapping over into the opposite direction in that it abuts on the one hand against the spacer pins and on the other hand engages the spring tongues.

Where the clutch is a pull-to-release clutch the diaphragm spring engages in the region of its outside diameter against the clutch housing and in the region of an intermediate diameter against the pressure plate through adjusting elements of the automatic wear adjuster, and the annular spring member has its outside diameter engaging in a housing gap which corresponds at least to the thickness of the material of the annular spring member, The annular spring member is then provided in its radially inner region with a plurality of circumferentially spaced tongues each of which extends through a gap between spring tongues of the diaphragm spring and engages against the rear face of spring tongues. This provides a particularly space-saving construction as the annular spring member extends substantially radially within the opening in the clutch housing.

In this arrangement the tongues of the annular spring member comprise hooks bent radially inwardly and spaced from the rear faces of the spring tongues of the diaphragm spring, and engaging with the diaphragm spring tongues through a circumferentially extending support ring. Such a construction has a simple construction and assembly and is secure in operation, even at high speeds.

The annular spring member is preferably secured with respect to the front faces of the diaphragm spring tongues through an engaging region interrupted solely by the tongues of the annular spring member. This ensures that the annular spring member cannot lose its effectiveness by snapping over from the initial installed position.

With such a construction the housing gap is advantageously formed by the internal edge of the clutch housing reduced by the extent of the thickness of the material of the annular spring member and by heads of securing rivets which are mounted to project radially inwards beyond the region of reduction in material thickness. Such a construction is simple to make and assemble.

In some constructions it can be advantageous to arrange the spring characteristic of the annular spring member so that it has a zero-crossing.

The release force applied by the annular spring member in the end region of the withdrawal travel is then equal to or greater than the force applied by the diaphragm spring to the withdrawal system. The diaphragm spring can then be returned to its engaged position by a forced action, for example such as is present in automatic clutch actuating systems.

it is however possible to alter the characteristic of the annular spring member in the end region of the withdrawal travel by additional means to avoid intersection with the characteristic of the diaphragm spring. In one embodiment the inside diameter of the annular spring member engages spring tongues of the diaphragm spring and the outside diameter engages the clutch housing, the clutch housing being so constructed and arranged that on transition from the clutch withdrawn state to the clutch engaged state the engagement between the annular spring member and the clutch housing moves from the outside diameter to a lesser intermediate diameter. This simple measure ensures that on the transition from the engaged to the withdrawn state the characteristic of the annular spring member is flattened in the desired region and an intersection cannot take place.

Advantageously the inside diameter of the annular spring member engages spring tongues of the diaphragm spring and the outside diameter engages the clutch housing and on movement from the clutch engaged state to the clutch withdrawn state the annular spring member has an intermediate diameter coming into engagement with the clutch housing. Such a construction is particularly simple to manufacture when the annular spring member is fixed at its outside diameter by spacer pins arranged concentric with the axis of rotation and at least one of the spacer pins has a radially inward extension against which the annular spring member can engage in the withdrawal sequence.

in a clutch in which the actuation of the diaphragm spring is by an hydraulic/pneumatic withdrawal system which has an axially fixed housing and an axially movable piston with a withdrawal bearing, the outside diameter of the annular spring member can in a particularly advantageous manner engage the housing and the inside diameter engages the piston.

Such a construction is particularly advantageous when the mounting of the annular spring member in the friction clutch itself, on whatever grounds, does not appear to be practicable.

Various embodiments of the invention are illustrated by way of example in the accompanying drawings, in which: Figure 1 is a longitudinal section through a friction clutch; Figure 2 is a longitudinal section through the upper half of a further clutch; Figure 3 shows a detail of Figure 2; Figure 4 shows the spring forces of the different components over the clutch withdrawal travel; Figure 5 is a longitudinal section through the upper half of a further friction clutch with the spring arranged in the withdrawal system; Figures 6 and 7 are longitudinal sections through the upper half of another friction clutch in the engaged and withdrawn conditions.

Figure 8 is a longitudinal section through the upper half of a further clutch; and Figure 9 shows the spring forces over the travel of the spring corresponding to Figures 5 and 8.

Figure 1 shows a friction clutch 1, of which the construction is known in principle for use in a motor vehicle. Thus a clutch housing 3 is rigidly attached to a flywheel (not shown) of an internal combustion engine and is rotatable with the flywheel about an axis 5. A pressure plate 6 is rotatable with the clutch housing 3 but is axially movable. A diaphragm spring 9 is located in the housing 3, and in the engaged position of the clutch applies an actuating force A to the pressure plate 6 so that the clutch plate 8 with its friction linings is clamped between the flywheel and the pressure plate 6 with the actuating force A. The diaphragm spring 9 engages in the region of its outer periphery against the pressure plate 6 and in the region of an intermediate diameter it engages against the clutch housing 3 through a number of circumferentially spaced spacer pins 16.

Radially inwards of this the diaphragm spring 9 is provided with individual circumferentially spaced spring tongues 11 engaged by a withdrawal system, not shown. The clutch 1 is a so-called push-to-release clutch in which the withdrawal system pushes the spring tongues 11 to withdraw the clutch.

Between the outside diameter region of the spring 9 and the pressure plate 6 there is an automatic wear adjuster 12 for the friction linings. The adjuster 12 includes adjusting elements 13 which, when wear occurs in the friction linings on the clutch plate 8, ensure that on displacement of the pressure plate 6 in the direction of arrow A with wear, the spacing between the pressure plate 6 and the radially outer region of the spring 9 is increased, dependent on the wear, so that over the entire working life of the friction linings on the clutch plate 8 the spring 9 can apply a constant actuating force A to the pressure plate 6. A more detailed description of the adjuster 12 will not be provided here, as there are various embodiments of it, such as can be seen for example in DE-A-35 18 781. The clutch 1 also has an annular spring member, such as a diaphragm spring or plate spring 14 which acts to reduce the forces applied for withdrawal of the clutch. This diaphragm or plate spring 14 is so constructed and arranged that with the clutch 1 in the engaged position illustrated it exerts little or no release force on the spring 9. On increasing withdrawal movement - that is a movement of the spring tongues 1 1 of the spring 9 in the direction of the arrow A and a movement of the radially outer edge of the spring 9 in the opposite direction - the diaphragm or plate spring 14 exerts an increasingly large withdrawal force on the diaphragm spring 9 so that the actuating forces for the clutch 1 can be significantly reduced.

Figure 4 shows a number of spring characteristics in relation to spring travel. The characteristic B for the diaphragm spring 9 shows the spring force in relation to the travel. It is a typical diaphragm spring characteristic with a rise in the spring force in the region of small travel and a fall away of the spring force with increasing spring travel. The point EB indicates the installed position of the spring in the clutch in the operating state with the clutch engaged. This installed position is held constant by the adjuster 12, and is chosen so that the actuating force produced in this state by the diaphragm spring 9 produces the clamping force for the clutch plate required to transmit the necessary torque. The withdrawal movement takes place from the installed position in the direction of greater spring travel. In Figure 4 the withdrawal movement runs from the point 5 and to about the points 6, 7. Between 6 and 7 the characteristic B reaches its minimum and then increases slightly. The diaphragm or plate spring 14 which is designed to reduce the withdrawal force has a characteristic C, which is the same in principle as the characteristic B but extends in the opposite direction. The spring 14 is installed in such a way that in the installed condition it exerts little or no release force on the spring 9. On increasing withdrawal movement the force increases rapidly as shown by the characteristic C and reduces by the same amount the actuating force which needs to be applied to the spring tongues 11 by the withdrawal system. Subtraction of the characteristic C from the characteristic B produces the characteristic D which represents the force to be applied by the withdrawal system. Account can also be taken of elasticities in the clutch housing 3, which is not absolutely rigid, and/or resilience between the friction linings on the clutch plate 8 which also affect the withdrawal force. Thus for example the spring characteristic E indicates the action of the lining resilience on the clutch plate 8. This represents a force opposing the spring characteristic B and can be subtracted from the spring characteristic D in the range of its effectiveness between the installed position and the point 6, to produce the spring characteristic F. This characteristic F is the actual withdrawal force that needs to be applied by the withdrawal system or by the driver. The spring characteristic F lies above the characteristic D in the region of the point 6 of the spring travel and then extends downwards with it. It can be clearly seen that there is a large difference between the spring characteristic B of the diaphragm spring 9 and the characteristic F which is to be applied to disengage the clutch. The clutch of Figures 1 and 4 can be actuated with very small forces. It should be noted that the tuning or matching of the installed position of the diaphragm or plate spring 14 is arranged so that it applies as small as possible a release force in the installed position. At this operating point therefore the diaphragm spring 9 is only lightly loaded. This is evident from Figure 4 in that the characteristics E and D start below the characteristic B, by the same amount by which in the installed position EB the characteristic C is above the line of zero spring force. However, in mass production of springs of this kind there is variation in the characteristics.

Although the aim is to keep the zero crossing of the spring characteristic C as near as possible to the installed condition, there could be combination of springs in which the zero crossing of the spring characteristic C is moved to the right, towards higher spring travels. In such a case the diaphragm or plate spring 14 would snap over in the engaged condition of the clutch 1 and would be put out of action. To avoid this the spring 14 in Figure 1 is arranged so that it cannot snap over into the inactive position, as it is held in the region of its radially inner diameter mechanically by the spring tongues 11 of the diaphragm spring 9 and at its radially outer diameter by the housing 3. The spring 14 abuts in the region of its outside diameter in a recess 19 which is formed between the inside of the clutch housing 3 and a corresponding edge of each spacer pin 16. The recess 19 corresponds substantially to the thickness of the material of the spring 14. The inside diameter of the spring 14 engages, through its own stress, over a circumferential engaging region 20 against the outsides of the spring tongues 11. The spring 14 has individual retaining elements 21 integral with the spring 14 and extending into the circumferential gap between two adjacent spring tongues 11. Each engages behind a spring tongue 11, that is, on its inner face. In this way the spring 14 cannot reach its inactive position and it augments the withdrawal force on every withdrawal sequence and with a spring force which increases.

Figure 2 shows a section through the upper half of a so-called pullto-release clutch 2: The clutch 2 has the same basic components as that of Figure 1, but arranged differently. Here the diaphragm spring 10 engages in the region of its outside diameter against the clutch housing 4 and at a smaller intermediate diameter against the pressure plate 7. The spring 10 continues radially inwards in the form of individual spring tongues 11 engaged by a withdrawal system, not shown. The pressure plate 7 is again rotatable with the clutch housing 4 but axially movable and it is urged by the diaphragm spring 10 with an actuating force A to clamp the clutch plate (not shown), between the pressure plate 7 and the flywheel (not shown). All the components of the clutch 2 are rotatable about the axis 5 with the flywheel.

An automatic adjuster 12 is arranged between the diaphragm spring 10 and the pressure plate 7 in the same way as in Figure 1.

in Figure 2 the spring tongues 11 of the diaphragm spring 10 are moved to the right, opposite to the direction of arrow A, in a withdrawal sequence. A diaphragm or plate spring 15 which assists the withdrawal force is arranged with its outside diameter against the clutch housing 4 and in the region of its inside diameter it extends in the form of a number of circumferentially spaced tongues 23 to the rear faces of the spring tongues 11, which it engages through a support ring 24. The engagement against the rear face of the diaphragm spring 10 is necessary because the release force applied by the spring 15 to the spring tongues 11 must be in the direction opposite to the arrow A. The opposing supporting force on the outside diameter of the spring 15 takes place at the clutch housing 4 in the region of its inside edge 26. The inside edge 26 of the clutch housing 4 has a reduced thickness at its external face by the thickness of the material of the spring 15. The outer diameter of the spring 15 is also secured against axial movement outside the housing by the heads 18 of securing rivets 17 which project radially inwards, and axially outside the clutch housing 4 as shown in Figure 3. The spring 15 is also fixed in the region of its tongues 23 by an engaging region 25 on the front faces of the spring tongues 11. The force relationship of the construction of Figures 2 and 3 is identical with the force relationship of the construction of Figure 1 and so is shown in Figure 4.

With increasing withdrawal movement the spring 15 exerts an increasing force corresponding to the spring characteristic C on the diaphragm spring 10 in a sense which reduces the actuating force. In this position its outer diameter engages the inner edge 26 of the clutch housing 4 and its inner diameter engages, through the tongues 23 and the support ring 24, against the rear faces of the spring tongues 11.

The heads 18 of the rivets 17 and the engaging region 25 on the front faces f the spring tongues 11 act to ensure it does not snap over into the nonactive position.

Figure 5 shows a friction clutch in which, differing from Figures 1 to 3, the annular spring 29 (a diaphragm or plate spring) is mounted outside the clutch. The clutch of Figure 5 is actuated by a withdrawal system which is operated pneumatically or hydraulically. The withdrawal system comprises a housing 33 mounted in a fixed position, for example on the wall of the vehicle gearbox and concentric with the axis 5. A piston 34 works in the housing and is also concentric with the axis 5. The piston 34 carries the withdrawal bearing or thrust race 28 which acts directly on the radially inner ends of the spring tongues 11 of the diaphragm spring 9. By admitting a pressure medium to the space between the housing 33 and the piston 34 the piston 34 actuates the diaphragm spring 9 through the withdrawal bearing 28. The annular spring 29 is arranged between the housing 33 and the piston 34. It abuts against the axially fixed housing 33 and its release force acts through the withdrawal bearing 28 on the diaphragm spring 9. In this construction it must be ensured that the path of the spring force of the diaphragm/plate spring 29 corresponding to the spring characteristic C in Figure 4 has a positive value in the region EB, regardless of whether it is also prevented from snapping into its inactive position, as in Figures 1 and 2. The arrangement of the spring 29 in the withdrawal system 27, means that a corresponding amount of space can be saved within the clutch itself.

Figures 6 and 7 show the engaged and withdrawn states of a friction clutch which is a modification of the clutch of Figures 1 and 2, and corresponding reference numerals have been applied to corresponding parts.

Thus, the clutch comprises a clutch housing 4 with a pressure plate 7 which is urged in the direction A by the force of a diaphragm spring 9 to clamp a clutch plate to a flywheel. An adjuster 12 is provided between the pressure plate 7 and the diaphragm spring 9. The spring 9 is mounted in the clutch housing 4 through spacer pins 16 arranged concentric with the axis 5, the spacer pins 16 predetermining the tilting circle on actuation of the clutch.

An annular spring in the form of a diaphragm spring 30 is arranged between the clutch housing 4 and the diaphragm spring 9 radially inwards of the spacer pins 16. The spring 30 acts in the same way as the springs 14 15.

Its inside diameter Dj engages the outside or front faces of the spring tongues 11 of the diaphragm spring 9 and in the region of its outside diameter it engages the clutch housing. The diaphragm spring 30 is centred in a radial direction by the spacer pins 16. In the region between the outside diameter Da of the diaphragm spring 30 and an intermediate diameter D, which is smaller than the diameter Da, an inclined face is provided on the clutch housing 4 in such a way that with the clutch in its engaged state the diaphragm spring 30 bears with its diameter D against the clutch housing 4 as shown in Figure 6 and with the clutch in its withdrawn state shown in Figure 7 it engages the housing with its outside diameter Da.

Figure 9 shows the spring characteristics of the clutch of Figures 6 and 7. As in Figure 4 the spring characteristic B is that of the diaphragm spring 9, while the spring characteristic E is that of the lining resilience of the clutch plate. The characteristic C is that of the spring 30, and it is very steep in the region between the installed position EB and the withdrawn state, resulting in a very small actuating force characteristic F' As a result there is a possibility that in the withdrawn state the spring characteristic C' will lie above the characteristic B of the diaphragm spring 9. Such an arrangement may be advantageous for automatic clutch actuation in which the clutch actuator can exert force on the diaphragm spring on both directions of movement of that spring (e.g. by engaging behind the diaphragm spring 9). In the clutch of Figures 6 and 7 however, despite the maintenance of the steep spring characteristic C' by the movement of the engagement point Da to D during the engaging sequence, it must be ensured that the characteristic C' does not intersect the characteristic B. Instead, the characteristic C" should be provided. The spring characteristic F starting from the installed state EB - then changes into the course of the spring characteristic D" and always remains in the positive region of spring force.

Figure 8 shows another way of influencing the spring force characteristic C' of the diaphragm spring 30 in such a way that it is flattened out with increasing withdrawal travel in accordance with C", so that intersection with the spring characteristic B is avoided. As in the clutch of Figures 6 and 7 the annular spring 30 is fixed in a radial direction by spacer pins 31 arranged concentric with the axis 5. At least one of these spacer pins 31 has a radially inward extension 32 on the side of the spring 30 adjacent the pressure plate 7. The extension 32 is spaced from the spring 30 with the clutch in its engaged state. Figure 8 shows the engaged statue, in which the spring 30 has its outside diameter Da engaging the clutch housing 4 and its inside diameter Dj engaging the spring tongues 11 of the diaphragm spring 9. If the clutch is now moved into its withdrawn state - by movement of the spring tongues 11 to the left - the spring 30 comes into engagement with the extension 32 after a part of the withdrawal movement, whereafter its effect on the diaphragm spring 9 decreases. This achieves the spring characteristic in accordance with C" and the withdrawal force remains in the positive region. In this arrangement the matching or tuning of the spring characteristic can be achieved through the engaging diameter D and the spacing between the extension 32 and the diaphragm spring 30.

The means described with reference to Figures 6 to 9 for influencing the spring characteristic of the diaphragm spring 30 can obviously be used in a construction in accordance with Figure 5. In a construction like Figure 5 it is then possible to make the spring characteristic C' steep in the region near the installed condition, reducing the withdrawal force to ensure that intersection with the spring characteristic B of the diaphragm spring 9 is avoided.

Claims

1. A friction clutch of the kind set forth, in which the diaphragm spring engages in an outer diameter region against the housing and in an intermediate diameter region against an automatic adjuster provided between the pressure plate and the diaphragm spring for takin gup the wear of the friction linings to maintain substantially constant an installed position of the diaphragm spring whereby the actuating force is substantially constant, and an annular spring member is provided having an outer diameter region engaging in a housing gap corresponding at least to the thickness of the material of the spring member and an inner diameter region provided with a plurality of circumferentially spaced tongues each of which extends through a gap between spring tongues of the diaphragm spring and engages the spring tongues on a rear face of the diaphragm spring, the annular spring member being so constructed and arranged to apply substantially no force in a clutch release direction when the clutch is engaged, and an increasing release force with increasing clutch withdrawal travel.

2. A friction clutch as claimed in claim 1, in which the release force applied by the annular spring member during the withdrawal travel is not greater than the force applied to the withdrawal system by the diaphragm spring.

3. A friction clutch as claimed in claim 1 or claim 2, in which the diaphragm spring engages in the region of an intermediate diameter against the pressure plate through adjusting elements of the automatic wear adjuster.

4. A friction clutch as claimed in any preceding claim, in which the tongues of the annular spring member comprise hooks bent radially inwardly and spaced from the rear faces of the spring tongues of the diaphragm spring and engaging with the diaphragm spring tongues through a circumferentially extending support ring.

5. A friction clutch as claimed in any one of claims 1 to 3, in which the annular spring member is secured with respect to the front faces of the diaphragm spring tongues through an engaging region interrupted solely by the tongues of the annular spring member.

6. A friction clutch as claimed in any preceding claim, in which the housing gap is formed by the internal edge of the clutch housing reduced by the amount of the thickness of the material of the annular spring member and by heads of securing rivets which are mounted to project radially inwards beyond the region of reduction in material thickness.

7. A friction clutch of the kind set forth, including an automatic adjuster for taking up the wear of the friction linings to maintain substantially constant an installed position of the diaphragm spring and the actuating force, and an annular spring member engaging an axially stationary component and another component of an actuating chain between and including the pressure plate and the withdrawal system, the annular spring member being so constructed and arranged to apply little or no force in a clutch release direction when the clutch is engaged, and an increasing release force with increasing clutch withdrawal travel.

8. A friction clutch as claimed in claim 7, in which the release force applied by the annular spring member during the withdrawal travel is not greater than the force applied to the withdrawal system by the diaphragm spring.

9. A friction clutch as claimed in claim 7 or claim 8, in which the annular spring member engages against the clutch housing and directly against the diaphragm spring.

10. A friction clutch as claimed in claim 9, in which the annular spring member is arranged substantially radially inwards of the radially inner engagement point of the diaphragm spring.

11. A friction clutch as claimed in claim 10, in which the annular spring member is secured in the region of its two engagement points in the direction of the force exerted by it and in the opposite direction.

12. A friction clutch as claimed in claim 11, in which the diaphragm spring engages in the region of its outside diameter against the pressure plate through adjusting elements of the automatic wear adjuster and is pivotally mounted in an intermediate diameter region on the clutch housing through spacer pins, and the annular spring member has its outside diameter engaging in recesses in the spacer pins which have a gap of at least the thickness of the material of the annular spring member with respect to the inside wall of the clutch housing.

13. A friction clutch as claimed in claim 12, in which a radially inner region of the annular spring member is engaged by an engaging region of spring tongues of the diaphragm spring, and the annular spring member has retaining elements engaging with the spring tongues to secure the spring member in the opposite direction.

14. A friction clutch as claimed in claim 11, in which the diaphragm spring engages in the region of its outside diameter against the clutch housing and in the region of an intermediate diameter against the pressure plate through adjusting elements of the automatic wear adjuster, and the annular spring member has its outside diameter engaging in a housing gap which corresponds at least to the thickness of the material of the annular spring member.

15. A friction clutch as claimed in claim 14, in which the annular spring member is provided in its radially inner region with a plurality of circumferentially spaced tongues each of which extends through a gap between spring tongues of the diaphragm spring and engages the rear faces of the spring tongues.

16. A friction clutch as claimed in claim 15, in which the tongues of the annular spring member comprise hooks bent radially inwardly and spaced from the rear faces of the spring tongues of the diaphragm spring and engaging with the diaphragm spring tongues through a circumferentially extending support ring.

17. A friction clutch as claimed in claim 15, in which the annular spring member is secured with respect to the front faces of the diaphragm spring tongues through an engaging region interrupted solely by the tongues of the annular spring member.

18. A friction clutch as claimed in any of claims 14 to 17, in which the housing gap is formed by the internal edge of the clutch housing reduced by the amount of the thickness of the material of the annular spring member and by heads of securing rivets which are mounted to project radially inwards beyond the region of reduction in material thickness.

19. A friction clutch as claimed in claim 7, in which the release force applied by the annular spring member in the end region of the withdrawal travel is equal to or greater than the force applied to the withdrawal system by the diaphragm spring.

20. A friction clutch as claimed in claim 19, in which the characteristic of the annular spring member is altered in the end region of the withdrawal travel by additional means to avoid intersection with the characteristic of the diaphragm spring.

21. A friction clutch as claimed in claim 20, in which the inside diameter of the annular spring member engages spring tongues of the diaphragm spring and the outside diameter engages the clutch housing, the clutch housing being so constructed and arranged that on transition from the clutch withdrawn state to the clutch engaged state the engagement between the annular spring member and the clutch housing moves from the outside diameter to a lesser intermediate diameter.

22. A friction clutch as claimed in claim 20, in which the inside diameter of the annular spring member engages spring tongues of the diaphragm spring and the outside diameter engages the clutch housing, and on movement from the clutch engaged state into the clutch withdrawn state the annular spring member has an intermediate diameter coming into engagement with the clutch housing.

23. A friction clutch as claimed in claim 22, in which the annular spring member is fixed at its outside diameter by spacer pins arranged concentric with the axis of rotation and at least one of the spacer pins has a radially inward extension against which the annular spring member engages in the withdrawal sequence.

24. A friction clutch as claimed in claim 7, in which actuation of the diaphragm spring is by an hydraulic/pneumatic withdrawal system having an axially fixed housing and an axially movable piston with a withdrawal bearing, and the outside diameter of the annular spring member engages the housing and the inside diameter engages the piston.

25. A friction clutch as claimed in any one of claims 7 to 24, in which the annular spring member comprises a diaphragm spring.

26. A friction clutch as claimed in any of claims 7 to 24, in which the annular spring member comprises a plate spring.

27. A friction clutch of the kind set forth substantially as described herein with reference to and as illustrated in Figures 1 and 4 of the accompanying drawings.

28. A friction clutch of the kind set forth substantially as described herein with reference to and as illustrated in Figures 2 and 3 of the accompanying drawings.

29. A friction clutch of the kind set forth substantially as described herein with reference to and as illustrated in Figures 5 and 9 of the accompanying drawings.

30. A friction clutch of the kind set forth substantially as described herein with reference to and as illustrated in Figures 6 and 7 of the accompanying drawings

3 1. A friction clutch of the kind set forth substantially as described herein with reference to and as illustrated in Figure 8 of the accompanying drawings.